The experiments proposed in this study investigate the hypothesis that tropomyosin phosphorylation (Tm-P) in the heart is altered by cardiac stress and this charge modification of Tm alters it's interactions within the thin filament to affect contraction. To date the modification of Tm resulting from cardiac stress and its effect on contraction are unknown. We propose three aims: I) To evaluate Tm-P in the stressed heart. Tm-P will be characterized in isolated perfused hearts treated with ischemia/reperfusion by phosphoprotein specific staining of 2-D gels and mass spectrometry. II) To determine the effect of Tm-P on binding to actin, troponin and to actin in the presence of troponin. Tm binding will be carried out by co-sedimentation and quantified on 2-D gels with densitometry. Ill) To determine the effect of Tm-P on the development of force in cardiac muscle. Calcium activated tension will be measured in skinned papillary bundles treated with gelsolin to selectively remove the thin filament and reconstituted with actin, troponin and non-phosphorylated or phosphorylated Tm. The knowledge gained from these experiments will provide novel insight into the regulatory mechanism of Tm-P and its effect on cardiac muscle contraction in health and disease.